It's a YMMV thing. In this case I think its easier to just pretend a few things happened slightly differently in "Balance of Terror" rather than to twist the meaning in the exact words to better fit the other shows. Each to their own.

Except that you're the one who's projecting a meaning into the BoT lines that simply is not there. The characters are not as completely dumbstruck by the very concept of invisibility as you claim. The exact words they speak make it clear that Starfleet understands the concept quite well and knows exactly how it could be done in theory, but simply never expected it to be made practical. So your interpretation is farther from the exact words than ours is.

But Enterprise showed us is was practical and known to be by Starfleet, hence the discontinuity and this entire discussion.